1. FIELD OF THE INVENTION
This invention relates to methods of and apparatus for maintaining glass in a molten state and partially conditioning molten glass for forming in the submerged throat between two containers for molten glass, such as a glass tank and a conditioning riser or box for wide range of pull rates.
2. DESCRIPTION OF THE PRIOR ART
Preparation of vitreous materials for formation into products involves establishing thermal conditions within the material which are of a controlled nature, usually throughout the delivered volume of material, and which fall in relatively narrow ranges. In the case of glass it has been known to melt glass constituents, for example cullet or batch, in a melter chamber in which the molten glass is raised to a temperature above that which is suitable for forming to the product sought. Frequently, the melter chambers are constructed and operated so that they deliver molten glass to the region from which it is withdrwn at temperatures in excess of those considered optimum for forming product.
In order to condition glass to the state from which it is formed it has been known to convey the material from the melter through a feed path which, for the designed rate of draw, extracts heat and brings molten glass to or near its forming condition. The thermal insulation for the path, degree of enclosure of the path, the thermal extraction effort on the glass in the path and the heat added to the glass in the path have been arranged to establish a thermal equilibrium for a given rate of flow which develops the desired forming condition. As the draw rate of molten glass is altered from the given rate, it has been suggested that the forming condition can be maintained at the delivery end of the feed duct by adjustment of the thermal extraction or adding means. Typical cooling has been accomplished by directing cooling air on the duct walls or directly on the upper surface of the glass where a free surface is available. Heating has been by means of fossil fuel fired burners in the duct and electrodes through which the energy for Joule effect heat is imposed.
Even with relatively fine tuning of thermal parameters of a heated feed path the range of adjustment has been limited to pull rates of between about fifty to one hundred percent of design capacity of the system since at the lower rates the transit time of the molten glass is sufficient to permit excessive cooling. Further, where system conditions have dictated an interruption of the forming or melting process the termination of flow in the feed path has resulted in the freeze-off of that path due to the heat loss. Such a freeze-off can be overcome in those areas where a free surface is available and fossil fuel burners can be employed to remelt the frozen glass. However in designs where the feed path includes a submerged throat no free sufrace is available and while attempts at removal of some refractory and remelting with burners has met with limited success, generally the freeze-off ends the campaign of the system.
Submerged throats have in some instances been subjected to Joule effect heating by locating electrodes beyond the throat at its opposite ends. In the usual Joule effect heated design there is insufficient electrical capacity to apply the requisite heat to the molten glass in the submerged throat by external electrodes. Typically a feed duct about eight feet long having a cross section about 12 inches high and 18 inches wide from a submerged throat in the melter to a riser conditioning box has been provided with electrodes located on the center line of the duct with one in the melter about a foot ahead of the throat and the other in the riser.
Electric heating of molten glass by Joule effect is complicated by the negative temperature co-efficient of resistance of the glass. As the glass temperature increases the resistance decreases to a degree that can create a runaway condition due to the increaed current and the squared current relationship to Joule effect heating. Conversely as the glass temperature decreases, a critical value can be achieved at which the decreased current, which can be imposed within the voltage limitations of the source of electrical power, reduces the Joule effect heating below that level which will sustain the melt temperature against the inherent heat loss of the mass and its constraining walls. In the case of Joule effect heated submerged throats in which heat loss exceeds the capacity to add heat, the drop in the temperature of the molten glass can enter a negative runaway condition wherein the glass is cooled to a temperature at which the available voltage to the electrodes in the melter and riser is insufficient to sustain glass temperature. Under these conditions the glass in the throat freezes effectively ending the campaign of the system.
Catastrophic cooling of an electrically heated submerged throat for a glass melting system can occur in normal operation of the system where the pull rate is reduced to a level at which the heat is extracted from the molten glass in that area at a rate which exceeds the capacity of the Joule effect heating system to maintain the necessary forming temperature at the feed path exit. It also can occur where there is a loss of electrical power for a substantial interval. In this latter instance the reduced heat capacity of the feeder path and its designed cooling construction causes the glass therein, particularly in a submerged throat to freeze before that of the melter and even if power is recovered before the melter glass cools below the critical temperature system campaign is ended.